Murderous cretins? Love the term.
You, as the GM, are totally in control of this. Creating the setting and the cultures is purely under your control. I will tell you from experience, if you create and use cultures with certain values, most players will work with it; and those that work agaainst them will do so from that position of their own volition. This is a ro9leplaying game; and you can make that part of the role they need to play.
Create tribal groups that people trade with, for example. Orcs in most of my settings, and other humanoid races, are tribal but do not eat their young or attack on sight. If your PCs are looked at with disdain for improperly lowering the boom, it teaches them. If they get thrown in jail for using ancient coinage or items that obviously came from someone's tomb, they'll be more circumspect. If the priest-types are told that they need to shrive the spirits of the dead opponents, it may not stop, but it may give them pause to see these things in a new light.
Similarly, creat the same kind of human drama as 'World in Motion' events for your players. Have the local gossip in the taverns be about a hanging judge, or have the local broadsheet carry gossip from a humanitarian mission to a goblin tribe that is suffering from the plague.
Hell, do a good enough job, and if the EXP or social rewards are there the PCs might try to go help save the goblins.
When players are younger, this is hard, but as they get older and deal with real life issues, injecting your setting with competing philosophies and religious tenets just adds depths to the world. It fosters a more realistic feel, which will have a synergistic effect of making more realistic, and less 2-dimensional PCs.